The present invention relates to a charged-particle beam irradiation method and system for performing a medical treatment such as a cancer treatment through irradiation with a charged-particle beam, and more particularly to a charged-particle beam irradiation method and system in which an affected part can be irradiated with a charged-particle beam in conformity of the shape of the affected part.
In the case where a cancer treatment is performed by use of a charged-particle beam such as a proton beam with a high energy generated by an accelerator or the like, it is required that an area having a diameter of about 20 cm should be irradiated with a proton beam having an energy of about 230 MeV at the highest. The conventional method for realizing this has been disclosed by W. T. Chu et al, "Instrumentation for treatment of cancer using proton and light-ion beams", Review of Science Instrument, Vol. 64, No. 8 (August 1993), pp. 2055-2122. In the disclosed method, an affected part is divided into a plurality of layers in the direction of depth in a body and is scanned layer by layer through irradiation with a charged-particle beam in conformity to the shape of each layer.
FIG. 9 shows the construction of a charged-particle beam irradiation system disclosed by the Chu et al's article. Referring to FIG. 9, a charged-particle beam 90 ejected from an accelerator is adjusted in energy by a degrader 17 so that the irradiation of a plurality of layers 210 to 212 in an affected part 202 of a body 201 with the adjusted beam is made in a sequence from a deeper layer to a shallower layer. The beam is scanned by use of first and second scanning electromagnets 31a and 31b which are disposed in the irradiation system so that the directions of deflection are orthogonal or vertical and horizontal in the plane of each layer.
The Chu et al's article has disclosed charged-particle scanning methods including a wobbler scanning method in which a beam is circle-wise scanned, a raster scanning method in which a beam is zigzag-wise scanned, and a pixel scanning method in which a beam is pixel-wise scanned. FIG. 10 shows a charged-particle beam irradiation method based on the raster scanning method. As shown in FIG. 10, a charged-particle beam 220 is zigzag-wise scanned in the first layer 210 in conformity to the shape of the first layer 210. A similar scanning is made in the n-th layer 212.
FIG. 11 shows a dose profile 230 (or a relationship between depth and dose) in the case where the irradiation is made with a charged-particle beam having a high energy and a dose profile 231 in the case where the irradiation is made with a charged-particle beam having a high energy. As shown in FIG. 11, the dose profile of the charged-particle beam has the value 240 or 241 of a dose peak called Bragg peak. A beam penetration depth providing the Bragg peak becomes larger as the energy is higher. It is also shown in FIG. 11 that the irradiation with the charged-particle beam is made with a small dose even at depth portions shallower than the Bragg peak providing portion. Referring to FIG. 10, this shows that when the irradiation with the charged-particle beam 220 is made for the first layer 210, a region 222 of the n-th layer 212 is also subjected to the irradiation with the same charged-particle beam 220. Accordingly, in the case where the irradiation with a charged-particle beam 221 is made for the n-th layer 212, it is required that the dose of a beam portion (indicated by dotted line) for irradiation of the region 222 should be reduced. Though only the first layer and the n-th layer are shown in FIG. 10 for simplification of illustration, the actual irradiation of the n-th layer amounts to the superimposed irradiation for the first to (n-1)th layers. Therefore, when the irradiation is to be made for the n-th layer, it is necessary that a dose for the beam portion indicated by dotted line in the n-th layer should be equal to or smaller than, for example, one tenth (at the largest ratio) as compared with a dose for a beam portion indicated by solid line.
For such requirements, the Chu et al's article has proposed two irradiation methods as follows. In a first method, the scanning speed of a charged-particle beam at the time of irradiation of each layer is constant while the intensity of the charged-particle beam is reduced when the region 222 is irradiated. In a second method, the intensity of a charged-particle beam at the time of irradiation of each layer is constant while the scanning speed of the charged-particle beam is increased when the region 222 is irradiated. With each of the first and second methods, it is possible to reduce the radiation dose of the charged-particle beam in the region 222.
In the first method, however, it is required that while one layer is being irradiated with a beam, the intensity of the beam should be changed greatly in accordance with an irradiation position. Namely, there is a problem that a large change in intensity of each charged-particle beam, for example, from 1 to 1/10 is needed in the period of 0.1 to 2 seconds when one layer is irradiated, which complicates the control of the accelerator ejecting the beam.
In the second method, it is required that the scanning speed of a beam at the time of irradiation of the region 222 should be increased to, for example, 10 times, which needs a large change in magnetic field intensity of the scanning electromagnet with time. Accordingly, there is a problem that a power supply voltage of the scanning electromagnet becomes high, thereby increasing the cost of a power supply for the scanning electromagnet.